1. Field of the Invention
This invention relates to a sensor which allows dielectric measurements to be made in a completely non-obstructive manner on a fluid such as molten plastic flowing in a conduit. In particular, it relates to a ceramic cylinder with an interdigitated electrode capacitor patterned into the inside wall. The sensor may be used for continuous determination of copolymer composition and polymer viscosity (molecular weight) of material flowing in a conduit.
2. Background
The dielectric properties of any polymer are frequency and temperature dependent functions of the dipolar and ionic atomic and molecular polarization. As a material is subjected to an alternating electric field, polar groups and free ions move and orient along the field lines, causing a buildup of charge at electrode surfaces. From measurements of the electrical current associated with this excess charge, the capacitance and dissipation of the system are computed. Using the exact geometry of the sensing volume, the dielectric constant and dielectric loss factor may be calculated. Dielectric properties can be used to measure polymer properties of interest such as thermal transitions, epoxy curing rates, water and additive concentrations, and voids in the polymer.
Such measurements can be made in a batch manner in the laboratory using flat capacitors, but to be useful as in-line sensors for application in the melt processing of thermoplastics, the capacitor should be made in a shape that can be installed in a conduit carrying molten polymer, and should be capable of reliable operation at the temperatures and pressures encountered with high-softening thermoplastics. The present invention provides such a sensor, preferably cylindrical, and shows it can be used in processing various molten polymers.
Dielectric spectroscopy is well known in the field of analytical chemistry. Interdigitated fringing field capacitors are also known, but the known devices are not suitable for use in a process pipeline
Fr. 2,342,838 makes capacitance measurements on molten polymer, but the sensor is of a different design from the present sensor, with coaxial electrodes employing a core mandrel and the outer wall of a tube.
U.S. Pat. No. 4,448,909 and U.S. Pat. No. 4,448,943 apply capacitance cell measurements at the outlet of an extruder to control process variables in a thermoplastic compounding process. In contrast with the present tubular design, the sensor is a single parallel plate dielectric cell attached to the output end of an extruder. This arrangement generates substantial back pressure in the extruder, has poor product flow in the dead zone between the plates, and incurs measurement error due to deflection of the plates.
U.S. Pat. No. 3,846,073 uses a side-stream capacitance sensor with coaxial electrodes employing a core mandrel and the outer wall of a tube. The sensor is used to measure the vinyl acetate content of an ethylene/vinyl acetate copolymer.
An interdigitated comb electrode is commonly used for obtaining dielectric measurements on surfaces of materials and fluids. Probes of this type have been used for many years as moisture detection devices. U.S. Pat. No. 3,696,369 discloses an interdigitated electrode for moisture sensing. In the past few years these interdigitated probe structures were adapted to measure dielectric properties of materials. See, for example, Society for the Advancement of Material and Process Engineering Journal, 19, No. 4, July/August, 1983.
U.S. Ser. No. 07/274,461, filed Nov. 21, 1988, discloses a planar dielectric sensor having interdigitated electrodes for off-line analytical work.
No reference is known to the inventors which teaches the use of an interdigitated electrode conduit structure for use as a flow-through measurement sensor for fluid streams.
The present work involves (1) the invention of a new cylindrical, interdigitated dielectric sensor for non-obstructive, real-time process measurements, (2) a technique for relating the real component of the complex permittivity (dielectric constant or capacitance) to copolymer composition, and (3) relating the imaginary component (dielectric loss factor or dissipation) to the melt viscosity or molecular weight of polymers, providing instantaneous measurements of the melt viscosity or molecular weight in polymer melt processing.
With the present technique, a process stream flows across the electrode surface, an alternating voltage is applied between the two "combs" of the "interdigitated electrode" pattern, and the resulting alternating current is measured. In this fashion the field penetrates the surface of the material to a depth 2-5 times the distance separating the fingers in the interdigitated electrode array.
An alternating electric field, applied to a polymer, orients permanent and induced dipoles and separates positive and negative ions in the polymer. The dipole or ion movement is a function of the applied conditions (temperature, pressure), thermal properties (melting point, phase transitions, heat of fusion), rheological properties (flow rate, viscosity), physical properties (density, molecular weight, degree of polymerization), optical properties (index of refraction), and chemical properties (composition, reaction rate constant, activation energy).
Following are some of the problems of the prior art which this invention solves.
a) The need to avoid the introduction of obstructions into the pipeline, which is accomplished by using an interdigitated fringing field capacitor on the inside wall of a conduit, preferably a cylinder.
b) The need to minimize the abrasion of the electrodes by the process fluids, which is accomplished by orienting the electrodes in the flow direction and by recessing the electrodes so they are substantially flush with the inside wall of the cylinder. In addition, the inside surface of the sensor may be coated with a 0.1 micrometer deposited layer of alumina which protects the electrodes and allows for easier cleaning.
c) The need to provide field penetration a substantial distance into the product stream, so the measurement extends beyond the layer of product that is close to the pipe wall and therefore moves very slowly. This is accomplished by using an unusually large spacing between electrodes. The tendency for large spacing to reduce capacitance is counteracted by using a large electrode area.
d) The need to remove the heat-sensitive amplifier equipment from the vicinity of the sensor so high temperature melts can be measured. This is accomplished by using a current-to-voltage converter and a lock-in amplifier.